In a simulcast paging or messaging system, data from a paging terminal are distributed to a plurality of transmitters for transmission to a receiving device that may be located anywhere within a relatively large service area of the system. Since the signal transmitted by each transmitter covers only a limited portion of the total service area, it is very possible that any of the receiving devices carried by users of the service will be located in overlap regions where the signals from two or more transmitters are received.
Conventional simulcast systems typically use two level frequency shift keying (FSK) to modulate the data transmitted by the plurality of transmitters. The paging/messaging simulcast systems in commercial use conform to one of the following standards: (1) the 512 baud (symbols/sec.) standard of the Post Office Code Standardization Advisory Group (POCSAG) also known as the CCIR Radiopaging Code No. 1 (RPC1); (2) the 600 baud Golay standard; (3) the 1200 baud POCSAG standard; or (4) the 2400 baud POCSAG. All of these systems use a 25 KHz channel and have a maximum efficiency of 2400/25000=0.096 bits/sec/Hz.
In a simulcast system using FSK modulation, differences in the propagation time for the signals from different transmitters reaching a receiving device located in an overlap region can cause degradation in the signal, which will increase the bit error rate (BER), because the sum of the signals from the different transmitters confuses the frequency discriminator or demodulator in the receiving device.
If the two RF signals have approximately equal power, which is a reasonable presumption for a receiving device in the overlap region, the resultant demodulated data bits are corrupted when the transmitted signals change from one frequency to another. The time during which the frequency of one transmitted signal overlaps with a different frequency from the other transmitted signal represents a random noisy portion of the received signal that causes a higher than desired BER, particularly, if the delay or difference in the propagation time of the different transmitted signals at the receiving device is greater than 1/4 of a baud duration. Noticeable improvement in the operation of a conventional simulcast paging system results if the delay times between transmissions at the receiving device are less than 1/4 of a baud. A typical maximum propagation time difference for signals from two adjacent transmitters in a simulcast system to reach a receiving device in the overlap region of the transmitters is about 54 .mu.sec. This delay occurs when the receiving device is 10 miles closer to one of the two transmitters than the other.
Another source of RF signal delay in a simulcast system is caused by differences in the time that the data to be transmitted reaches each of the transmitters from the central paging terminal. These timing errors can readily be controlled at the transmitters or at the paging terminal to equalize the time for the signal from the paging terminal to reach the transmitters, to within approximately .+-.10 .mu.sec. Thus, for two transmitters, the worst case delay of this type is 20 .mu.sec., including a contribution of 10 .mu.sec. from each of the transmitters. In this typical example, the worst case total delay time, including the time delay due to lack of synchronization between the signals transmitted from two adjacent transmitters and the delay caused by differences in the time for the two signals to reach a receiving device on the edge of the overlap region between the two transmitters is thus 20+54=74 .mu.sec.
If the limit of acceptable delay is 1/4 baud, the minimum baud duration, T, is simply 4.times.74=296 .mu.sec., and the maximum baud rate for a conventional simulcast system is 1/T=1/296 .mu.sec.=3378 baud (symbols per second). Conservatively, the maximum baud rate of a simulcast system is generally limited to about 3000 baud by this requirement to limit the effects of delay on the combined signals received from plural transmitters in a simulcast system.
Other sources of degradation that can adversely affect a simulcast signal include the Raleigh fading that often occurs when the receiver is moving, e.g., when the receiver is in a moving vehicle. Errors in the received signal can also arise due to slight differences in the transmission frequencies of simulcast base stations that are supposed to be transmitting nominally identical signals to a receiver. These differences in frequency cause distortion that can degrade the quality with which the transmitted RF signal is received. Each of these different sources of error can thus degrade the "quality" of the received signal. As used herein, the term "quality of the received signal" is intended to encompass degradations caused by: (a) different delays affecting multiple received signals; (b) relative differences in the carrier frequency of multiple transmitters; (c) Raleigh fading rate and fading characteristics; (d) relative differences in signal gain and phase between the signals received from multiple transmitters; and (e) noise level or signal-to-noise ratio (SNR). The term of art used herein to represent or define the quality of a received signal (except for degradation in the received signal caused by noise) is "channel impulse response" (CIR).
Recently, an improved simulcast data rate and modulation standard was proposed by the European Radio Message System (ERMES). This standard is a 4-level FSK scheme having a baud rate of 3125 and providing a data rate of 6250 bits/sec.--only slightly better than the baud rate limitation of more conventional simulcast systems. Another system, which was proposed by the Telocater committee, also has a data rate of 6250 bits/sec. and is a variation of the ERMES system. This system will likely be introduced into commercial use in North America in about one year. For the 6250 bits/sec. data rate, the efficiency is limited to 0.25 bits/sec./Hz. Both of these four-level FSK systems, as well as the more conventional two-level FSK system now in use, belong to a class of modulation referred to as constant envelope modulation.
The Federal Communications Commission (FCC) has solicited proposals for communications systems that might increase the efficiency of the RF spectrum utilization. In response, paging and messaging companies have submitted proposals for advanced systems to substantially increase the data rate of communication systems over current contemplated standards, including those noted above. Of the proposals submitted, one by Mobile Telecommunications Technologies (MTEL), consisting of two documents entitled, "PETITION FOR RULEMAKING," Nov. 13, 1991 and "REQUEST FOR PIONEER'S PREFERENCE," Nov. 12, 1991, described a modulation of 8 tone on-off keying (a 256 level scheme) with a baud rate of 3000. This scheme has a data rate of 24,000 bits/sec. on a 50 KHz channel; its efficiency is thus 24000/50000=0.48 bits/sec./Hz--twice that of the ERMES standard and the highest efficiency of a simulcast system in the prior art, but still relatively low.
Accordingly, it will be apparent that further improvements in the baud rate of simulcast communications are desirable. A simulcast communications system that overcomes the apparent limitation imposed by the 1/4 baud delay that can occur in the synchronization and propagation times of simulcast transmissions is required. The improvement should be accomplished by using techniques that do not simply increase the number of levels of modulation.